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Focusing an outside view (light) down a tube then expanding it

  1. Feb 11, 2012 #1
    Can someone please tell me if there is name for focusing the outside light to a point down a tube then magnifying it back showing the outside view?

    E.g. having the lenses focus on a hill side then focus down a 1 meter tube then enlarge onto wall.

    If there is no name for this what type of lenses do I need to make it?

  2. jcsd
  3. Feb 11, 2012 #2


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    welcome to pf!

    hi tim999! welcome to pf! :wink:

    that's just an ordinary convex lens

    the distance from the lens to the wall must be equal to the focal length of the lens, or the image will be blurred

    but the image won't be very large, it'll be the focal length times tanθ, where θ is the angle subtended (at your eye) by the hillside

    (and the larger the image, the fainter it will be)

    you might be better off turning your room into a camera obscura, using a pinhole instead of a lens …

    see http://en.wikipedia.org/wiki/Camera_obscura :smile:
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